As is well known in the art, an AWD (All Wheel Drive) vehicle can be provided with at least one hydraulic disc coupling for distributing the driving torque from the vehicle engine to all of the vehicle wheels. Especially, such a coupling may be provided in the drive line between the front axle and the wheels of the rear axle, most often in the vicinity of the rear axle differential.
Sometimes, it may be desirable to use the AWD vehicle in a FWD (Forward Wheel Drive) mode. In this case the coupling is disconnected, i e its discs are separated from each other.
By operating the vehicle with the coupling in a disconnect mode, rotating masses in the vehicle are reduced, leading to a lower fuel consumption.
In a practical case a propeller shaft of the vehicle, possibly including one or two bevel gear transmissions, has a hydraulic disc coupling at one end and a simple coupling, for example a dog clutch, at the other end. In the disconnect mode both these couplings must be disconnected for obtaining the desired effect.
When the AWD mode is resumed, the coupling must resume its connected state very quickly, say within a period of 0.4-0.5 s or less. For several reasons such a short time is difficult to obtain.
In order to secure proper disconnection, the play between neighboring discs in the coupling must be in the order of typically 0.08-0.10 mm, so that the drag torque from the coupling is minimized. This means that the required movement of a hydraulically operated piston of the coupling has to be considerable.
If use is made of a pump actuator system, such as is disclosed in WO 2011/043722, in which the supply of hydraulic oil to the coupling piston is governed by the rotative speed of a pump, it will be extremely difficult to obtain the required fast and long movement of the coupling piston without unduly increasing the pump displacement by say three to five times.
Another problem to be solved is that the speed-up torque during the connection sequence is low (typically 5-10% of the maximum AWD torque) but has to be accurate. As the accuracy of the torque is mainly proportional to the maximum torque (pressure) in the AWD mode, the percentage tolerance will be high in the low torque range.
This second problem cannot be easily solved in a pump actuator system without inventive measures.